Re: [Xen-devel] Question about VPID during MOV-TO-CR3

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.09.16 at 12:35, <tamas.lengyel@xxxxxxxxxxxx> wrote:
> On Sep 22, 2016 02:56, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>>
>> >>> On 21.09.16 at 17:30, <tamas.lengyel@xxxxxxxxxxxx> wrote:
>> > What I'm saying is that the guest OS should be in charge of managing
>> > its own TLB when VPID is in use. Whether it does flush the TLB or not
>> > is not of our concern. If it's a sane OS it will likely flush when it
>> > needs to, but we should not be jumping in and doing it as we do right
>> > now. We are actually breaking the architectural behavior by forcing a
>> > flush, MOV-TO-CR3 doesn't by itself flush the TLB on real hardware.
>>
>> I continue to not understand where you take this from. Writes to
>> CR3 have always been doing TLB flushes - full ones prior to the
>> introduction of global pages, and flushes of only non-global entries
>> nowadays. In fact prior to the introduction of INVLPG and CR4
>> there was no other way to flush TLBs.
> 
> Yes, I meant it doesn't completely flush the TLB as we do right now when
> invalidating the whole VPID.

But then what architectural behavior do you see broken? Flushing
more than is required is always permitted. (And again - I'm all for
improvements here, we just need to be careful to not remove
flushing that is architecturally required.)

Jan


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